The field is internal combustion engines. Particularly, the field relates to two-stroke internal combustion engines. More specifically, the field covers exhaust management strategies for opposed-piston, two-stroke engines.
As seen in FIG. 1, an opposed-piston engine includes at least one cylinder 10 with a bore 12 and longitudinally-displaced exhaust and intake ports 14 and 16 machined or formed therein. Fuel injector nozzles 17 are located in or adjacent injector ports that open through the side of the cylinder, at or near the longitudinal center of the cylinder. Two pistons 20, 22 are disposed in the bore 12 of the cylinder with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred as the “exhaust” piston because of its proximity to the exhaust port 14; and, the end of the cylinder wherein the exhaust port is formed is referred to as the “exhaust end”. Similarly, the piston 22 is referred as the “intake” piston because of its proximity to the intake port 16, and the corresponding end of the cylinder is the “intake end”.
Operation of an opposed-piston engine with one or more cylinders such as the cylinder 10 is well understood. In this regard, and with reference to FIG. 2, in response to combustion occurring between the end surfaces 20e, 22e the opposed pistons move away from respective top dead center (TDC) positions where they are at their closest positions relative to one another in the cylinder. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions in which they are furthest apart from each other. The pistons may move in phase so that the exhaust and intake ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
In many opposed-piston constructions, a phase offset is introduced into the piston movements. As shown in FIG. 1, for example, the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BDC positions in a sequence in which the exhaust port 14 opens as the exhaust piston 20 moves through BDC while the intake port 16 is still closed so that combustion gasses start to flow out of the exhaust port 14. As the pistons continue moving away from each other, the intake port 16 opens while the exhaust port 14 is still open and a charge of pressurized air (“charge air”) is forced into the cylinder 10, driving exhaust gasses out of the exhaust port 14. The displacement of exhaust gas from the cylinder through the exhaust port while admitting charge air through the intake port is referred to as “scavenging”. Because the charge air entering the cylinder flows in the same direction as the outflow of exhaust gas (toward the exhaust port), the scavenging process is referred to as “uniflow scavenging”.
As the pistons move through their BDC locations and reverse direction, the exhaust port 14 is closed by the exhaust piston 20 and scavenging ceases. The intake port 16 remains open while the intake piston 22 continues to move away from BDC. As the pistons continue moving toward TDC (FIG. 2), the intake port 16 is closed and the charge air in the cylinder is compressed between the end surfaces 20e and 22e. Typically, the charge air is swirled as it passes through the intake port 16 to promote scavenging while the ports are open and, after the ports close, to mix the air with the injected fuel. Fuel (which is, typically, diesel) is injected into the cylinder by one or more high pressure injectors. With reference to FIG. 1 as an example, the swirling air (or simply, “swirl”) 30 has a generally helical motion that forms a vortex in the bore that circulates around the longitudinal axis of the cylinder. As best seen in FIG. 2, as the pistons advance toward their respective TDC locations in the cylinder bore, fuel 40 is injected through the nozzles 17 directly into the swirling charge air 30 in the bore 12, between the end surfaces 20e, 22e of the pistons. The swirling mixture of charge air and fuel is compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e when the pistons 20 and 22 are near their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations.
Each complete movement of a piston 20, 22 (TDC to BDC and BDC to TDC) is a “stroke”. Since one complete power cycle of the engine occurs in two complete movements of a piston, the engine is referred to as a “two-stroke engine” or a “two-stroke cycle engine”.
As illustrated in FIG. 2, fuel is directly injected through the side of the cylinder (“direct side injection”) into the cylinder bore and the movement of the fuel interacts with the residual swirling motion of the charge air in the bore. As the engine operating level increases and the heat of combustion rises, an increasing amount of nitrogen oxide (NOx) is produced. However, increasingly stringent emission requirements indicate the need for a significant degree of NOx reduction. One technique reduces NOx emission by exhaust gas recirculation (“EGR”). EGR has been incorporated into spark-ignited four-stroke engine constructions and two-stroke, compression-ignition engines with a single piston operating in each cylinder. EGR constructions for opposed-piston, two-stroke engines are taught in the assignee's U.S. patent application Ser. No. 13/068,679, filed May 16, 2011 and published as US 2011/0289916 A1 on Dec. 1, 2011.
However, while opposed-piston two-cycle engines can now be equipped with EGR to reduce NOx emissions, new control strategies are needed to manage tailpipe emissions (exhaust) in response to varying engine operating conditions such as cold start, low load, and low ambient temperature.
In this regard, diesel engines can be equipped with after-treatment systems that subject exhaust gasses to catalytic processes that convert HC, CO and NOx. The catalytic materials must be heated to a “light-off” level in order to function. After-treatment systems may also include a diesel particulate filter (DPF) that filters soot from the exhaust gasses. Thermal energy must be delivered to the filtration material in order for it to achieve a temperature level at which it properly regenerates. In both cases, heat is obtained from the exhaust gasses themselves. However, when the engine is initially turned on from an ambient non-operating thermal state (referred to as a “cold start”), the exhaust gasses are insufficiently hot to activate the catalytic materials and/or to regenerate the DPF. Presently, the majority of tailpipe emissions during certification as well as under real-world driving conditions occur just after starting the engine, before the catalyst and filtration materials reach operating temperatures. In many applications, greater than 50% of the tailpipe emissions on a diesel FTP-75 test occur in the cold start phase (Lambert, C., 2006. “Advanced CIDI Emission Control System Development”, Report for the U.S. Department of Energy, DE-FC26-01NT41103, Jun. 30, 2006). In fact, with a well designed after-treatment system, it can be shown that greater than 50% of the tailpipe emissions occur during the first 200 seconds of the test. Thus, the time from initial engine start-up to when the catalyst and filtration materials have enough heat to perform adequately needs to decrease.
Accordingly, it is desirable to operate an opposed-piston two-cycle engine during cold start by rapidly raising the exhaust gas temperature in order to achieve early activation of after-treatment materials.
Another characteristic of diesel cold start is combustion instability in which successful combustion is punctuated by complete or partial misfires as the engine accelerates to an idling speed. Therefore, in some aspects, it is desirable that a strategy for elevating exhaust temperatures during cold start of an opposed-piston two-stroke engine also provide for combustion stability.
In other words, it is desirable to provide high exhaust temperatures in the exhaust gasses of an EGR-equipped, opposed-piston two-stroke engine under varying operating conditions while maintaining the engine's combustion stability.